This invention generally relates to data processing system storage facilities and more specifically to a motor control circuit for tape drive units used in tape cassette magnetic storage facilities.
Magnetic tape cassette storage facilities for use in data processing systems have become very popular in recent years. This popularity stems from several factors. For example, the cassette is self-contained; thus, it is easier to handle the tape in a cassette than in a reel-to-reel tape storage facility. Tape cassettes and their drives are relatively simple and reliable. Moreover, the facilities are less expensive and more compact than reel-to-reel tape facilities.
Normally a tape cassette storage facility contains one or more drive units connected to a controller. The controller produces various control signals to start and stop the tape and otherwise to control data transfers to or from the tape. A conventional drive unit for use in such a facility includes motor driven spindles for moving the tape past openings in a cassette housing and past a transducer mounted on the drive. During a reading operation, the transducer transmits electrical signals in response to the magnetic patterns on the tape. During a writing operation, the transducer alters the magnetic patterns on the tape in response to the incoming electrical signals. Edge and corner blocks and other elements properly position the cassette housing with respect to the transducer. Various sensors monitor the presence of the tape cassette in the drive unit and the appearance of "end-of-tape" apertures in the tape for use by other circuits associated with the drive unit and controller.
It is important in these facilities that the tape in a drive unit pass the transducer at a constant speed. Some drive units attain constant tape speed by means of a constant speed drive, commonly a capstan, which engages the tape. In others, the tape contains a pre-recorded clock track. A control circuit receives clock pulses derived from the clock track and uses them to control motor, and thus tape, speed. In one such control circuit, the clock pulses modulate reference pulses from a constant frequency oscillator. In other such control circuits, to which this invention is primarily directed, the clock pulses are converted to an analog input signal for a direct-current drive-motor speed servo circuit.
In one such servo circuit for a tape cassette drive unit, the control circuit transmits direction signals in response to signals from the controller. These direction signals designate the direction of tape motion and thereby define one drive motor as a "driving" motor. The other drive motor is then a "driven" motor. The motors rotate in opposite directions when energized independently. They both connect to the output of a servo power supply controlled by the drive-motor speed servo circuit. Switching elements, which respond to the direction signals, connect one motor to ground as the driving motor. Thus, if the control circuit transmits a "forward" direction signal, the forward drive motor switching element closes to energize the forward motor as the driving motor with the output of the drive-motor speed servo circuit. The drive-motor speed servo circuit receives an analog signal dependent upon the repetition rate of pulses corresponding to each clock pulse and compares this analog signal with a dc reference signal derived from a diode junction.
It is also desirable to move the tape under tension. In the foregoing circuit, diodes from each junction formed by a motor and its corresponding switching element connect to a common junction controlled by a tension servo circuit that controls the power supplied to the drive thereby to maintain the tape under tension. The switching element which closes on the driving motor bypasses its corresponding diode but the other diode is coupled to a negative power supply through the tension servo circuit thereby to partially energize the driven motor. Thus, during normal operations, the torque generated in the driven motor opposes the motion produced by the driving motor and maintains tension on the tape.
The drive unit interprets the absence of both direction signals as a stop command. In the foregoing control circuit, a dynamic braking circuit responds to the stop command by disabling the switching element connected to the driving motor. Simultaneously, the braking circuit enables the fixed-width clock pulses derived from the clock track periodically to close the switching element connected to the driven motor, thereby to energize fully the driven motor on an intermittent basis. As a result, the tape slows so the periods between the energization of the driven motor lengthen. Thus, the average power to the driven motor decreases; and the tape slows and eventually stops.
Certain aspects of these servo control and other circuits have several deficiencies in commercial applications. For example, the reference voltage for the drive-motor servo circuit is derived from a diode junction. Although the reference voltage from a diode junction tends to be independent of power supply voltage variations, it is sensitive to variations in junction temperature. This temperature-caused instability detracts from the desirable constant speed characteristics. Moreover, variations in the diode junction voltages among individual diodes require a calibration circuit such as a potentiometer arrangement to provide a proper reference voltage. The introduction of a potentiometer arrangement complicates the circuit and increases its cost.
Other problems can occur during a start-up operation. When the control circuit transmits a direction signal, the drive-motor speed servo circuit receives the full reference voltage and transmits a maximum error signal. Thus, the servo circuit energizes the driving motor, which initially is at rest, at a maximum level; and the motor accelerates rapidly. It then is possible for the driving motor to accelerate to beyond the desired velocity; i.e., enter an "overspeed" condition. If this occurs, the normally positive common junction of the tension diodes can become negative causing the diode connected to the driving motor to bypass the corresponding switching element. During these conditions the drive motor servo circuit can become ineffective and both tension diodes can turn on. This is an unstable condition.
In use, the tape often is moved intermittently to read or write one or more records in succession. No reading or writing operations occur during the intervals that the tape accelerates to or decelerates from its normal operating speed. It is generally assumed that the tape accelerates to its proper operating speed within a fixed startup interval. However, friction characteristics of the tape cassette vary widely. These characteristics can largely determine the distance that the tape actually travels during the startup interval. If the actual acceleration varies from the assumed constant acceleration, then, obviously, the spaces between adjacent records on the tape differ. Thus, this characteristic reduces the average density of data on the tape including inter-record gaps.
High accelerations during start-up also can produce erratic tape motion. Consequently, the tape can slap or flutter against the transducer, thereby to damage the tape and cause various tape sensors positioned near the transducer to transmit improper signals.
When the stop command is received, the driving motor in the prior circuit is de-energized while the driven motor receives current pulses, as previously described. When a supply spool or reel on the driven motor has a relatively high inertia in comparison with the inertia of the take-up spool on the driving motor, the supply spool can continue to rotate even after the tape stops advancing past the transducer. Loose tape accumulates within the cassette under this condition. When the drive subsequently is energized, the driving and driven motors rotate in opposite directions until the tape snaps taut. This action can break the tape.
Moreover, this drive has an upper limit for reading data from the tape. The reading circuit contains a gain circuit and a peak detector comprising differentiator and threshold circuits. Output data signals from the peak detector are sampled periodically. In this drive, however, the sample timing is dependent upon the differentiator output signals which undergo wide voltage excursions and therefore delay sample timing. As a result, the sample is taken after an optimum time which corresponds to the instant the signal from the transducer should have a peak value. This reduces the maximum reading speed which can be attained. Moreover the threshold circuit is susceptible to electrical noise, so erroneous data signals can result.
Therefore, it is an object of this invention to provide an improved electrical control circuit for a magnetic tape cassette drive unit;
Another object of this invention is to provide a magnetic tape cassette drive unit control circuit which includes a more stable reference voltage for control purposes;
Another object of this invention is to provide a magnetic tape drive unit control circuit in which maximum reading speeds from the tape are improved;
Still yet another object of this invention is to provide a magnetic tape drive unit control circuit which is less susceptible to noise during reading operations.